The assembly of electronic products and more specifically the permanent assembly of electronic components to printed circuit boards has involved the use of some form of relatively low-temperature solder alloy (e.g., tin/lead or Sn63/Pb37) since the earliest days of the electronics industry. The reasons are manifold but the most important one has been the ease of mass joining of thousand of electronics interconnections between printed circuit and the leads of many electronic components.
Lead is a highly toxic substance, exposure to which can produce a wide range of well known adverse health effects. Of importance in this context, fumes produced from soldering operations are dangerous to workers. The process may generate a fume which is a combination of lead oxide (from lead based solder) and colophony (from the solder flux). Each of these constituents has been shown to be potentially hazardous. In addition, if the amount of lead in electronics were reduced, it would also reduce the pressure to mine and smelt it. Mining lead can contaminate local ground water supplies. Smelting can lead to factory, worker, and environmental contamination.
Reducing the lead stream would also reduce the amount of lead in discarded electronic devices, lowering the level of lead in landfills and in other less secure locations. Because of the difficulty and cost of recycling used electronics, as well as lax enforcement of legislation regarding waste exports, large amounts of used electronics are sent to countries such as China, India, and Kenya, which have lower environmental standards and poorer working conditions.
Thus, there are marketing and legislative pressures to reduce tin/lead solders. In particular, the Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (commonly referred to as the Restriction of Hazardous Substances Directive or RoHS) was adopted in February 2003 by the European Union. The RoHS directive took effect on Jul. 1, 2006, and is required to be enforced and become law in each member state. This directive restricts the use of six hazardous materials, including lead, in the manufacture of various types of electronic and electrical equipment. It is closely linked with the Waste Electrical and Electronic Equipment Directive (WEEE) 2002/96/EC which sets collection, recycling and recovery targets for electrical goods and is part of a legislative initiative to solve the problem of huge amounts of toxic electronic device waste.
RoHS does not eliminate the use of lead in all electronic devices. In certain devices requiring high reliability, such as medical devices, continued use of lead alloys is permitted. Thus, lead in electronics continues to be a concern. The electronics industry has been searching for a practical substitute for tin/lead solders. The most common substitutes in present use are SAC varieties, which are alloys containing tin (Sn), silver (Ag), and copper (Cu).
SAC solders also have significant environmental consequences. For example, mining tin is disastrous both locally and globally. Large deposits of tin are found in the Amazon rain forest. In Brazil, this has led to the introduction of roads, clearing of forest, displacement of native people, soil degradation, and creation of dams, tailing ponds, and mounds, and smelting operations. Perhaps the most serious environmental impact of mining tin in Brazil is the silting up of rivers and creeks. This degradation modifies forever the profile of animal and plant life, destroys gene banks, alters the soil structure, introduces pests and diseases, and creates an irrecoverable ecological loss.
Worldwide ecological problems stemming from mismanagement of Brazil's environment are well known. These range from pressures on global warming from the destruction of rain forest to the long term damage to the pharmaceutical industry by the destruction of animal and plant life diversity. Mining in Brazil is simply one example of the tin industry's destructive effects. Large deposits and mining operations also exist in Indonesia, Malaysia, and China, developing countries where attitudes toward economic development overwhelm concerns for ecological protection.
SAC solders have additional problems. They require high temperatures, wasting energy, are brittle, and cause reliability problems. The melting temperature is such that components and circuit boards may be damaged. Correct quantities of individual alloy constituent compounds are still under investigation and the long term stability is unknown. Moreover, SAC solder processes are prone to the formation of shorts (e.g., “tin whiskers”) and opens if surfaces are not properly prepared. Whether tin/lead solder or a SAC variety is used, dense metal adds both to the weight and height of circuit assemblies.
Therefore there is a need for a substitute for the soldering process and its attendant environmental and practical drawbacks.
While solder alloys have been most common, other joining materials have been proposed and/or used such as so-called “polymer solders” which are a form of conductive adhesive. Moreover, there have been efforts to make connections separable by providing sockets for components. There have also been electrical and electronic connectors developed to link power and signal carrying conductors described with various resilient contact structures all of which require constant applied force or pressure.
At the same time, there has been a continual effort to put more electronics into ever smaller volumes. As a result, over the last few years there has been interest within the electronics industry in various methods for integrated circuit (IC) chip stacking within packages and the stacking of IC packages themselves, all with the intent of reducing assembly size in the Z or vertical axis. There has also been an ongoing effort to reduce the number of surface mounted components on a printed circuit board (PCB) by embedding certain components, mostly passive devices, inside the circuit board.
In the creation of IC packages, there has also been an effort to embed active devices by placing unpackaged IC devices directly inside a substrate and interconnecting them by drilling and plating directly to the chip contacts. While such solutions offer benefits in specific applications, the input/output (I/O) terminals of the chip can be very small and very challenging to make such connections accurately. Moreover the device after manufacturing may not successfully pass burn in testing making the entire effort valueless after completion.
Another area of concern is in management of heat as densely packaged ICs may create a high energy density that can reduce the reliability of electronic products.